1. Field of the Invention
The present invention relates to a ferrite core suitable for measures for common-mode noise of various kinds of electronic devices in which a high-frequency signal is used, its manufacturing method, and a common-mode noise filter used in a differential transmission circuit and the like.
2. Description of the Related Art
A common-mode noise filter is used for measures for unnecessary radiation of a power supply line or measures for a common-mode noise of a high-frequency signal.
The common-mode noise filter is constituted such that a plurality of conductor wires are wound around a ferrite core 906 by several turns to some dozen turns. According to the conventional example disclosed in JP-A 2002-329618, as shown in FIGS. 3A–3C, the ferrite core 906 includes a ferrite porcelain having a wound core 904, flanges 901 provided at both ends thereof, and a plurality of legs 902 continued to the flanges 901, and electrodes 903 are formed on top surfaces of the respective legs 902. According to the conventional example shown in FIGS. 3A–3C, the plurality of conductor wires are wound around the wound core 904 of the ferrite core 906 constituted as described above by several turns to some dozen turns, and starting ends of the conductor wires are connected to the electrodes 903 on the top surfaces of the legs 902 by soldering, thermo-compression or the like, and finishing ends thereof are connected to the electrodes 903 on the top surfaces of other legs 902 by soldering, thermo-compression or the like.
According to this conventional example, the electrode 903 is constituted such that a paste for a thick film conductor such as Ag, AgPd and the like is applied to respective top surfaces of the legs 902 by a method such as dipping, screen printing, transferring or the like and burned, and several layers formed of Ni or Cu, Sn, SnPb, Au and the like are formed on the thick film conductor depending on usage or requests.
The plated film is formed on the thick film conductor by dipping a ferrite porcelain on which the thick film conductor is formed, into a plating solution containing predetermine metal and applying a current. In addition, after the plated film is formed, the plating solution attached onto the ferrite porcelain is rinsed out to be removed.
According to the thus constituted common-mode noise filter, when a common-phase current is applied to two conductor wires, a magnetic flux is reinforced and impedance is increased. Meanwhile, when a reversed-phase current is applied to the two conductor wires, the magnetic flux is negated and impedance is decreased. Thus, the common-mode noise filter is an electronic component having a filter function in which the common-phase current flows little but the reversed-phase current flows well.
In addition, in a field of information communication devices in which the common-mode noise filter is used, there are demands for smaller and lighter components. According to the demands, an almost rectangular size of the area when mounted becomes gradually smaller, that is, from 3216 type in which the size is 3.2 mm by 1.6 mm, to 2520 type in which the size is 2.5 mm by 2.0 mm and further to 2012 type, 1608 type and 1210 type.
In addition, as shown in FIGS. 3A–3C, JP-A 2002-329618 discloses that all boundary parts between the top surfaces and side faces of each leg 902 in the ferrite core 906 have curved surfaces.
In addition, a curvature radius of the curved surface body of the leg 902 is 0.2 to 0.3 mm and the side faces of the four legs 902 on the side of the wound core 904 are inclined surfaces 908 inclined from the upright direction by 30 to 70°.
According to the above, since the boundary part between the top surface and the side face of the leg 902 has the curved surface whose curvature radius is about 0.2 mm, it is assumed that breaking or short-circuit of the conductor wire at this boundary part can be prevented. Furthermore, since the side face of the leg 902 on the side of the wound core 904 has the inclined surface 908, the conductor wire wound around the wound core 904 by bifilar winding can be connected to the electrode 903 at a gentle angle. As a result, it is shown that there can be provided the common-mode noise filter having higher reliability.
According to the ferrite core according to a second conventional example disclosed in Japanese Patent No. 3168972, as shown in FIGS. 15A, 15B and 15C, an inclined surface is formed at a leg 952.
According to the second conventional example, it is assumed that since a conductor wire 957 can have a necessary distance because of the inclined surface, a short-circuit, pressure deterioration and the like caused when an electrode 954 of the leg 952 to be connected comes into contact with an electrode 954 of the adjacent leg 952 can be sure prevented.
In addition, it is assumed that when the conductor wire 957 is mounted along the inclined surface, since a connection angle of the conductor wire 957 can be gentle, the conductor wire 957 can be prevented from breaking so that there is provided a common-mode noise filter having higher reliability.
However, in the filed of information communication devices in which such common-mode noise filter is used, there are demands for further smaller and lighter components in accordance with miniaturization of the device. As a result, according to a wiring type of common-mode noise filter, its almost rectangular size when mounted is gradually miniaturized from a size of 3.2 mm by 1.6 mm to a size of 2.5 mm by 2.0 mm, 2.0 mm by 1.2 mm, 1.6 mm by 0.8 mm, and 1.2 mm by 1.0 mm.
Under such circumstances, the common-mode noise filter using the ferrite core disclosed in JP-A 2002-329618 has the following problems.
That is, when the electrode 903 is formed on the top surface of the leg 902, there is a problem such that the plating elongates along a boundary part (vertical corner portion) between the side faces of the leg.
When the plating elongates along the vertical corner portion, it becomes difficult to acquire insulating properties as the component becomes smaller. That is, as shown in FIGS. 4 and 5, when the electrode 903 is formed on the top surface of the leg 902, the plated layer is liable to elongate along the edge line of the leg and it becomes difficult to acquire insulating properties.
According to a structure of the conventional common-mode noise filter, in the case of the size of 3.2 mm by 1.6 mm, for example, even when the plating of the electrode 903 elongates as described above, since the distance between electrodes 903 of the legs 902 is kept at about 0.6 mm, insulating properties can be remained, but when the size becomes smaller, it becomes difficult to acquire the insulating properties.
In addition, although it is preferable to increase the thickness of the electrode 903 in order to acquire mounting strength when the common-mode filter is mounted on a substrate, since the plating elongates along the vertical corner portion in the conventional structure, there is a limitation in forming thickly the plate layer and it is difficult to sufficiently acquire the mounting strength. That is, the plating elongates well as the thickness of the plating is increased.
Furthermore, as shown in FIG. 6, there is a problem such that the conductor wire connected to one electrode 903 of the adjacent legs comes in contact with the plated layer elongated along the vertical corner portion of the other leg to cause a short-circuit.
Furthermore, since the dimensions of the electrodes 903 vary because of the elongation of the plating, there is a problem such that loss of a magnetic flux generated at the ferrite core 6 varies and a Q value (loss characteristic) is liable to vary.
Still further, according to the common-mode noise filter using the ferrite core disclosed in Japanese Patent No. 3168972, there is a problem such that the area of the top surface of the leg 952 is decreased because the inclined surface is provided at the leg 952, so that adhesion strength when mounted is lowered.